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Lithium Distribution in Aluminum-Free Cubic Li7La3Zr2O12 Hui Xie,† Jose A. Alonso,‡ Yutao Li,§ Maria T. Fernandez-Díaz,^ and John B. Goodenough*,† †
Materials Science & Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain § State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China ^ Institut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9, France ‡
bS Supporting Information KEYWORDS: lithium garnet, Li7La3Zr2O12, solid electrolyte, neutron diffraction imitation of the flammable organic liquid-carbonate electrolytes of the Li-ion battery have stimulated interest in Li+-ion solid electrolytes having a conductivity σLi > 10 4 S cm 1 that are stable on contact with a lithium anode on one side and with water of variable pH on the other side.1 The “stuffed” Li garnets are promising candidates.2 4 The garnet structure A3B3C2O12 contains a B3C2O12 framework structure of B cations in 8-coordination sites and C cations in octahedral sites.5,6 The framework contains a 3D-connected interstitial space consisting of the tetrahedral 24d-A sites bridged by a single octahedron sharing on opposite sides a common face with each of the two neighboring 24d-A sites. The existence of only face-sharing sites, 3 tetrahedral A and 6 bridging octahedral sites for a total of 9 sites per formula unit, can provide a low activation energy for motion of guest Li+ ions if the Li+ ions are disordered with partial occupancies of both sites. The Li3Nd3Te2O12 garnet contains Li+ guest ions well-ordered into the A sites7,8 with a significant tetrahedral-site preference;9 it has a σLi≈10 7 S cm 1. However, additional Li+ ions can be “stuffed” into the interstitial space as in Li5La3M2O12 (M = Nb, Ta)10 12 and Li5+xMxLa3-xTa2O12 (M = Ca, Sr, Ba; 0 < x e 1.6).13 15 A previous neutron-diffraction study of the system16,17 has provided insight into how the Li+-ion occupancies of the interstitial space changes with Li concentration. The highest Li+ion conductivity, σLi = 2.4 10 4 S cm 1, has been reported for a nominal Li7La3Zr2O12 that was stable at the T = 1230 C needed for sintering a dense ceramic,3 a stability made possible by the adventitious incorporation of Al3+ impurity ions.18 We have prepared an Al-free nominal Li7La3Zr2O12 that decomposes easily above 850 C via a tetragonal phase. In order to find out the Li distribution in the Li-stuffed garnet structure and to clarify how the Li+-Li+ interactions limit the concentration of Li+ ions in the interstitial space and influence σLi, we have undertaken a neutron-diffraction determination of the Li+-ion site occupancies of our Al-free nominal Li7La3Zr2O12 to supplement the previous neutron diffraction study16,17 of garnet structures with 5 6.6 Li per formula unit. In a previous study, O’Callaghan and Cussen16,17 showed that the Li+-Li+ interaction across a shared site face displaces the octahedral-site Li+ from the 48 g position to a 96 h position near the opposite face, as is illustrated in Figure 1b. This observation tells us that
L
r 2011 American Chemical Society
Figure 1. (a) 3D connection of Li sites within the interstitial space of the garnet framework with 7.5 Li per formula unit. (b) Displacement of Li+ from central 48 g to 96h position in a bridging octahedral site with only one 24d Li+ neighbor. (c) Loop structure and the separations of Li atoms.
the Li+ ions cannot occupy an octahedral site that has a Li+-ion on both face-sharing A sites; displacement of the Li+ ion from a 48 g to a 96 h site by one tetrahedral-site Li+ will then displace to another neighboring octahedral site any tetrahedral-site Li+-ion adjacent to an occupied 96 h site. This deduction leads to the prediction of a theoretical upper limit for the Li concentration of 7.5 per formula unit, but this limit requires half-occupancy of the 24d-A tetrahedral sites with an ordering of the A-site vacancies and a full occupancy of the octahedral sites, as shown in Figure 1a. With disordered A-site vacancies, a practical upper limit would be xm < 7.5 per formula unit. An Al-free nominal Li7La3Zr2O12 sample with cubic symmetry was synthesized at low temperature as described in the Supporting Information. 7Li2CO3 was used to reduce the problem with the high neutron absorption cross-section of 6Li. X-ray diffraction showed formation of a cubic structure with space group Ia3d and lattice parameter a = 13.0035 Å; a small amount of La2O3 impurity was also visible. To identify the Li+-ion positions, a roomtemperature neutron-diffraction pattern was collected with the high-resolution D2B neutron diffractometer at ILL- Grenoble, Received: June 13, 2011 Revised: July 20, 2011 Published: July 28, 2011 3587
dx.doi.org/10.1021/cm201671k | Chem. Mater. 2011, 23, 3587–3589
Chemistry of Materials
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Figure 2. Observed (dots), calculated (line), and difference (bottom) neutron-diffraction patterns of Al-free cubic Li7La3Zr2O12 at room temperature. The markers show the peak positions of all possible Bragg reflections of cubic Li7La3Zr2O12, Li2CO3, and La2O3, from top to bottom.
Table 1. Refined Structure Parameters of Cubic Li7La3Zr2O12 at Room Temperature Uiso/Ueq x
atom site occupation
Y
Z
(Å2)
Li1 24d 0.564(12) 3/8 Li2 96 h 0.442(3) 0.6802(8)
0 0.5968(8)
1/4 0.1004(9)
0.026(2) 0.034(3)
La
24c
1
1/8
0
1/4
0.0124
Zr
16a
1
0
0
0
0.0107
O
96 h 1
0.28209(10) 0.10070(11) 0.19449(13) 0.0174
France. Li2CO3 was also detected in the neutron diffraction pattern; it derived from the excess Li2CO3 used in the synthesis to compensate for Li loss during sintering. The impurity phases were introduced in the structure refinement. The refinement was initiated with the garnet framework having La, Zr, and O located at 24c-B sites, 16a-C sites, and 96 h-O sites, respectively. Fourier maps to locate the Li+ within the framework identified two positions that can be assigned to the 24d-A sites and the displaced 96 h bridging octahedral sites, as shown in Figure S2. Attempts to introduce Li into the 48 g octahedral sites resulted in a negligible occupancy and did not improve the refinement quality; a Li+ ion in a 48 g site would require the two neighboring 24d-A sites be empty. Therefore, we conclude that all the Li+-ions in the bridging octahedral sites have one occupied and one empty near-neighbor 24d-A site and that the bridging octahedral sites between two filled 24d-A sites are empty to give a 24d-A site occupancy >0.5 and a bridgingoctahedral site occupancy
